1. Field of the Invention
This invention relates to a process of manufacturing inner and outer parts for a rotary piston machine in which said parts have parallel axes and interengaging helical engaging surfaces having convolutions differing in numbers by one and are operable to perform rotating and eccentric revolving movements relative to each other. At least one of said parts is machined by a cutting tool, which is rotated about an axis which is parallel to and spaced from the axis of the part that is to be machined.
2. Description of the Prior Art
A rotary piston machine of such special type will be obtained if the helical engaging surfaces of the cooperating inner and outer parts, are screwed together in such a manner that the closed individual chambers remaining between the inner and outer parts will move along the axes of the parts in case of a corresponding relative movement. Such machines may be used as pumps, which are known as Mohno pumps and serve to handle particularly liquids having a relatively high viscosity, and when energized by a liquid flow can be used as motors, e.g., for a rotary drive of drilling heads and the like. The relative movement may be imparted to the inner and outer parts in three different ways: the outer part may be stationary and the inner part may rotate and may eccentrically revolve about the axis of the outer part; or the inner part may be stationary and the outer part may rotate and may eccentrically revolve about the axis of the inner part, or the inner and outer parts having axes spaced apart from each other by a certain eccentricity may rotate at the same time.
In the manufacture of the inner and outer parts of such rotary piston machines it is particularly difficult to machine the outer part, which has an internal engaging surface which is accessible only with difficulty for being machined by conventional turning and milling tools. For this reason it is conventional to prefabricate a core, which permits the outer parts to be cast. But in that case an elastic material, such as rubber, is used or the outer part must be split. When known cutting tools are used to machine the engaging surface, such tools will be required to form not only the respective cross-sections out of the blank but they must also rotate the cross-section in steps during the longitudinal feed movement to provide the helical engaging surface. For this reason only very low feed rates are permissible and the manufacturing times are very long. Besides, the stability of the tool when cutting will be low because owing to the geometrical relationships between the cross-section and the helical shape and the resulting small clearance space the shank of the tool must be very thin so that it has only a low flexural and torsional stiffness. If the ratio of the diameters between the cross-section to be formed and the milling tool employed exceeds a certain value, a machining of the engaging surface of the outer part will not be possible even in theory because in that case the tool shank will have to be smaller than zero. Owing to the low stability of the tool when cutting it is also virtually impossible to manufacture relatively long parts and such a manufacture will require that sufficiently short parts be joined by adhesion, screwing, welding and the like. Besides, the machining of the cavity of the outer part in lavers will inevitably result in stepped traces formed by the machining so that the feed rate will be further restricted and the desired surface finish will be adversely affected.
Various machines are known for a machining of the external helical engaging surfaces of the inner parts and such machines perform a whirling operation for manufacturing single-convolution engaging surfaces of inner parts having a screwed circular cross-section or for making inner parts having engaging surface having one or more convolutions use a profile-milling cutter, which is guided along the lead of the desired profile, or a numerically controlled standard milling cutter. Such machines can only be used to machine external surfaces but cannot be used to machine internal surfaces.